Tonometry

Test Overview

A tonometry test measures the pressure inside your eye, which is called intraocular pressure (IOP). This test is used to check for glaucoma, an eye disease that can cause blindness by damaging the nerve in the back of the eye (optic nerve). Damage to the optic nerve may be caused by a buildup of fluid that does not drain properly out of the eye.

Tonometry measures IOP by recording the resistance of your cornea to pressure (indentation). Eyedrops to numb the surface of your eye are used with most of the following methods.

Tonometry methods

Applanation (Goldmann) tonometry. This type of tonometry uses a small probe to gently flatten part of your cornea to measure eye pressure and a microscope called a slit lamp to look at your eye. The pressure in your eye is measured by how much force is needed to flatten your cornea. This type of tonometry is very accurate and is often used to measure IOP after a simple screening test (such as air-puff tonometry) finds an increased IOP.

Electronic indentation tonometry. Electronic tonometry is being used more often to check for increased IOP. Although it is very accurate, electronic tonometry results can be different than applanation tonometry. Your doctor gently places the rounded tip of a tool that looks like a pen directly on your cornea. The IOP reading shows on a small computer panel.

Noncontact tonometry (pneumotonometry). Noncontact (or air-puff) tonometry does not touch your eye but uses a puff of air to flatten your cornea. This type of tonometry is not the best way to measure intraocular pressure. But it is often used as a simple way to check for high IOP and is the easiest way to test children. This type of tonometry does not use numbing eyedrops.

Why It Is Done

Tonometry may be done:

As part of a regular eye examination to check for increased intraocular pressure (IOP), which increases your risk of glaucoma.

To check the treatment for glaucoma. Tonometry can be used to see if medicine is keeping your IOP below a certain target pressure set by your doctor.

How To Prepare

Tell your doctor if you or someone in your family has glaucoma or risk factors for glaucoma.

If you wear contact lenses, remove them before the test. Do not put your contacts back in for 2 hours after the test. Bring your eyeglasses to wear after the test until you can wear your contact lenses.

Loosen or remove any tight clothing around your neck. Pressure on the veins in your neck can increase the pressure inside your eyes. Stay relaxed.

How It Is Done

Tonometry takes only a few minutes to do.

Applanation (Goldmann) method

This type of tonometry is done by an ophthalmologist or an optometrist. Your doctor will use eyedrops to numb the surface of your eyes so that you will not feel the tonometer during the test. A strip of paper containing a dye (fluorescein) will be touched to your eye, or eyedrops containing the dye will be applied. The dye makes it easier for your doctor to see your cornea.

You will rest your chin on a padded support and stare straight into the microscope (slit lamp). Your doctor sits in front of you and shines a bright light into your eye. Your doctor gently touches the tonometer probe to your eye. Your doctor checks the tension dial on the tonometer that measures the IOP of your eye.

Do not rub your eyes for 30 minutes until the numbing medicine has worn off.

Electronic indentation method

Electronic tonometry can be done by a technician, an optometrist, an ophthalmologist, or a family medicine doctor. Your doctor will use eyedrops to numb the surface of your eyes so that you will not feel the tonometer during the test.

You will stare straight ahead, or sometimes look down. Your doctor gently touches the tonometer probe to your eye. Several readings will be taken on each eye. You will hear a clicking sound each time a reading is obtained. After enough accurate readings have been obtained, a beep will sound, and the averaged IOP measurement will appear on the instrument's display panel.

Do not rub your eyes for 30 minutes until the numbing medicine has worn off.

Noncontact (or air-puff) method

This type of tonometry is done by an ophthalmologist or an optometrist. You do not need drops to numb your eye for this method.

You will rest your chin on a padded support and stare straight into the machine. A brief puff of air is blown at your eye. You will hear the puffing sound and feel a coolness or mild pressure on your eye. The tonometer records the intraocular pressure (IOP) from the change in the light reflected off the cornea as it is indented by the air puff. The test may be done several times for each eye.

How It Feels

Tonometry should not cause any eye pain. Your doctor will use eyedrops to numb the surface of your eyes so that you will not feel the tonometer during the test. You may have a scratchy feeling on your cornea. This usually goes away in 24 hours.

Some people become anxious when the tonometer needs to be touched to the eye. In air-puff tonometry, only a puff of air touches the eye.

Risks

There is a very slight risk that your cornea may be scratched during the methods that involve touching a tonometer to your eye. Rubbing your eyes before the numbing eyedrops wear off increases the risk of scratching the cornea. If tonometry causes a scratch on the cornea, your eye may be uncomfortable until the scratch heals, which normally takes about a day.

There is also a very small risk of an eye infection or an allergic reaction to the eyedrops used to numb your eyes.

With the air-puff (noncontact) method, there is no risk of scratches or infection, since nothing but air touches your eyes. But this method is not the best way to measure intraocular pressure.

You should not have any eye pain or vision problems after tonometry. Call your doctor if you feel any eye pain during the test or for 48 hours after the test.

Results

A tonometry test measures the pressure inside your eye, which is called intraocular pressure (IOP). This test is used to check for glaucoma.

Normal eye pressure is different for each person and is usually higher just after you wake up. IOP changes more in people who have glaucoma. Women usually have a higher IOP than men, and IOP normally gets higher as you get older.

High values

A high IOP may mean that you have glaucoma or that you are at high risk for developing glaucoma. People who have ongoing pressures above 27 mm Hg usually develop glaucoma unless the pressure is lowered with medicines.

People who have an ongoing IOP higher than 21 mm Hg but do not have optic nerve damage have a condition called ocular hypertension. These people may be at risk for developing glaucoma over time.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

Having a sore on your eye or an eye infection. This increases your risk of an eye injury during the test.

Being extremely nearsighted, having an irregularly shaped cornea, or having had major eye surgery in the past.

Blinking or squeezing your eyes shut during the test.

Having had laser refractive surgery (such as LASIK).

What To Think About

Tonometry tests may be done over months or years to check for glaucoma. Also, because intraocular pressure (IOP) can change at different times of the day, tonometry is not the only test done to check for glaucoma. If the IOP is high, more tests, such as ophthalmoscopy, gonioscopy, and visual field testing, may be done.

Pachymetry uses ultrasound to measure the thickness of the cornea. The thickness of the cornea can affect IOP measurement. Pachymetry is often done during a tonometry test. It can help your doctor know your chance for developing glaucoma.

Normal IOP is different from person to person. About 40% to 50% of people who have optic nerve damage caused by glaucoma have normal IOP.2, 3 In some cases of glaucoma there is damage to the optic nerve even though the eye pressure is never above normal.

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